Refactor out initialization pattern for Maven as WITH-AETHER to be

used by external consumers/manipulators of the Maven Aether libraries.
Unfinished, but can be used now to guarantee that ABCL-ASDF has
located Maven, which will be retrofitted compatible with current
usage.
TODO Introduce toplevel handler for finding Maven.

(error"Please obtain and install maven-3.0.3 or later locally from <http://maven.apache.org/download.html>, then set ABCL-ASDF:*MVN-LIBS-DIRECTORY* to the directory containing maven-core-3.*.jar et. al."))

;; FIXME Remove warning; put message in restart

(warn"Please obtain and install maven-3.0.3 or later locally from <http://maven.apache.org/download.html>, then set ABCL-ASDF:*MVN-LIBS-DIRECTORY* to the directory containing maven-core-3.*.jar et. al.")